Three-phase controllers use the principle of phase gating to control the electrical power which is supplied to an electrical load, in particular an asynchronous machine. In the case of asynchronous machines, this control of the power supply makes it possible to reduce the starting currents and the starting torque, and thus to achieve so-called soft starting. This so-called trigger delay α, which is also referred to as a delay is used as a measure for control purposes. The load which is connected to the output of the three-phase controller has current half-cycles of alternate polarity flowing through it with a time period in which no current flows and which is governed by the phase gating being located between each two successive current half-cycles.
The power sections of three-phase controllers are normally equipped with three sets of thyristors connected back-to-back in parallel. Since the costs for the thyristors are, however, the cost-driving factor of the entire appliance as the rating of the three-phase controller increases, three-phase controllers are also used having only two sets of thyristors connected back-to-back in parallel. In these so-called two-phase three-phase controllers, the remaining outer conductor is in the form of a conductor which cannot be connected.
One side effect of this two-phase three-phase controller of simplified design is that the root mean square values of the currents in the two controlled outer conductors are different, despite the same phase gating angles being used. This effect is caused by the magnetic interaction between the three outer conductor currents in the driven asynchronous machine, and is dependent on the rotation direction of the power supply system and/or of the asynchronous machine.
Because the current magnitudes are not the same, the power semiconductors (thyristors) as well as their heat sinks must be designed for higher values than necessary since it is necessary to cope with the respectively higher current. Which of the two currents is higher in the controlled outer conductors depends on the rotation direction of the connected power supply system, and can accordingly vary. This effect has been taken into account in the past by designing the power semiconductors such that each of the two valve sets in the two controlled outer conductors can carry the higher current.